The limit-current oxygen probe will be, for tile sake of brevity, referred to in the following as simply an oxygen probe. The function of the oxygen probe for measuring the oxygen content of a lean mixture will be explained below with respect to FIG. 5.
In FIG. 5, the pump current ip is shown plotted as a function of the pump voltage Up at a pregiven operating temperature. This operating temperature can, for example, be 850.degree. C. with the ohmic internal resistance of the probe being 100 .OMEGA.. As soon as tile pump voltage is applied to the probe, O.sub.2 molecules, which are entrained in the exhaust gas penetrating into the diffusion space of the cell, are reduced to O.sup.2- ions. These O.sup.2- ions are pumped through the electric field applied to the probe material and out of the diffusion space. For low pump voltages, tile current is limited only by the ohmic resistance. For this reason, the ip-Up characteristic is at first linear. However, when only very few O.sub.2 molecules are in the exhaust gas, then a saturation of the pump current occurs already at relatively low pump voltages. This is shown in FIG. 5 by the solid line which occupies the lowest position in this graph. The higher the saturation level is, the leaner is the mixture.
The pump voltage is typically pregiven with such a great value that the pump current is in the saturation range for actual use of such a probe in the flow of the exhaust gas of an internal combustion engine. In this way, this current is a direct measure for the oxygen content of the exhaust gas penetrating into the diffusion space of the probe. For the sake of completeness, it should be noted that measurement methods also exist which vary the pump voltage in dependence upon the oxygen content, namely, the lower the oxygen present, the lower the pump voltage. From FIG. 5, it can be seen directly that for lower quantities of oxygen, lower pump voltages can be used, but the condition is still satisfied that measurements are made in the saturation region of the characteristic. For the invention, it is insignificant whether the pump voltage is held constant or is varied in dependence upon oxygen content.
The characteristics shown in FIG. 5 by the solid lines apply to a probe newly placed into service. However, the internal resistance of a probe increases with increasing deterioration. To provide an overview in FIG. 5, it is assumed that the internal resistance after long service is only half of the internal resistance at the start of use with the temperature being the same in each case. As shown by the broken line in FIG. 5, the pump current is greatly limited by ohmic resistance and the pump current still does not reach its saturation for a very lean mixture for the pump voltage, which is applied in the practical application and is shown in FIG. 5 by a vertical line. Accordingly, the limit current is not measured which is determined by the actual oxygen concentration; instead, a lower current is measured which means that the oxygen concentration is incorrectly measured. In order to prevent this, the probe is controlled to a constant internal resistance of the electrochemical cell. In this way, the slope of the described characteristic is maintained even during deterioration of the probe but the probe temperature increases with increasing probe deterioration.
In addition to internal resistance, the magnitude of the output signal of a probe is, however, also greatly dependent upon temperature because the diffusion conditions differ greatly with temperature. In order to exclude measurement errors caused by temperature, it is known to control the temperature so that the probe temperature is constant. This control is effected via a measurement of the internal resistance of the probe's heater (U.S. Pat. No. 4,708,777). It is also known to measure the probe's temperature and to correct the measurement signal of the probe, which indicates the oxygen content, with the aid of the measured temperature (German patent publication 3,840,248).
The foregoing applies essentially also to two-cell oxygen probes wherein O.sup.-2 ions are pumped out of a diffusion space or into this diffusion space via a pump cell in order to measure a lean or rich mixture. The pumping takes place in such a manner that a second cell, the sensor cell, always shows a pregiven constant voltage. With deterioration, such a probe also shows an increase of the internal resistance with the danger that the pump voltage which is available is no longer adequate to carry out the total actually required O.sup.-2 transport. This difficulty can be avoided when a constant internal resistance is adjusted; however, then here too the problem of incorrect measurement occurs because of temperature increase.
The invention is directed to amperometric oxygen probes and these probes include single-cell and two-cell oxygen probes having the above-described functions.